This invention relates to infrared gas analyzers and more particularly to such gas analyzers with an automatic zero adjustment. Still more particularly it relates to such gas analyzers having multiple channels using a programmable microprocessor for performing the zero adjustment.
Infrared gas analyzers typically utilize an infrared source to direct infrared energy through a mixture of unknown gases contained in a sample cell. The infrared energy passing through the mixture in the sample cell at one or respective frequencies is measured as indicative of the relative concentrations of respective gases in the mixture, the respective gases exhibiting different absorption characteristics at respective frequencies. That is, the absorption by respective gases is frequency dependent.
One such analyzer is described in Passaro, et al., U.S. Pat. No. 4,346,296. As there described, an infrared source emits infrared at relatively constant intensity over a relatively broad spectrum. The infrared radiation from the source is interrupted periodically by a chopper. After passing through the sample cell, the chopped infrared is detected by respective detectors. In each case the radiation is filtered by a narrow band-pass filter so that such detector is effectively sensitive only to radiation of a particular narrow band of frequencies corresponding to a respective absorption frequency characteristic of a respective gas. The respective detection signals are thus systematically related to the relative concentration of the respective gases. Because of the chopper, these signals are AC signals at the chopper frequency. The signals are amplified, detected (rectified) and filtered to produce corresponding DC signals. Each filtered signal is applied to one input of a so-called span amplifier, which comprises a summing amplifier with an offset reference input and a controllable feedback input. The offset offsets the zero point, and the feedback controls span or the magnitude of input signal required for full scale output. The signal input has a gain control to control the point at which the signal input balances out the offset signal and hence the zero point of the span amplifier. That is the point where there is zero output from the span amplifier in the absence of absorption of the incident infrared radiation. The output of the span amplifier is proportional to the difference between the detection signal (after the gain control) and the offset signal. It may be noted that difference includes the addition of signals of opposite polarities. The amplifier output is measured by a meter, which may be a recorder, which is calibrated to indicate the relative concentration of the respective gas.
As described in the aforesaid U.S. Pat. No. 4,346,296, the zero point of the span amplifier is adjusted, that is, the instrument is zeroed, by introducing a so-called zero gas into the sample cell and adjusting the gain control of the signal input to provide a zero meter reading. The zero gas is a gas, such as nitrogen, which is substantially nonabsorptive of infrared radiation, at least at the frequencies passed by the respective filters. When a predetermined calibrating gas is introduced into the sample cell, the gain of the feedback signal is adjusted to some predetermined calibrated value. Then when the gas to be analyzed is introduced, the output meter properly records or indicates the relative concentrations of the respective constituent gas. For the purposes of checking the span calibration a push-button switch was operated to put a fixed input into the summing span amplifier and the resultant meter reading set at a predetermined level by adjusting the span control in the feedback loop.